It is widely expected that proteomic research will greatly facilitate the discovery of novel tumor targets. Major advances have been made in the identification of targets for diagnostic purposes. However, limitations of the present technologies have hindered identification of new therapeutic targets. The techniques commonly employed in proteomics, such as two-dimensional gel electrophoresis, Liquid Chromatography Tandem Mass Spectrometry (LC/MS/MS), Matrix-Assisted Laser Desorption Ionization/Mass Spectrometry (MALDI-MS), and the yeast two-hybrid system have not met the demand for “drugable” targets, such as cell surface markers.
Tumor targeting antibodies and peptides can be isolated by library display approaches (e.g. Aina, 2002; Hoogenboom, 1998). This is usually accomplished by screening a phage display library, or libraries with other display formats, against purified tumor specific or tumor associated antigens. However, tumor targeting antibodies and peptides have also been isolated by panning libraries against tumor cells or tumor tissues without prior information on the molecular targets. Noteworthy advantages of the latter approach are: (i) the isolated antibodies/peptides bind to native forms of their antigens/ligands on the cell surface whereas purified tumor antigens are often recombinant in nature and lack post-translational modification, (ii) the antigens are accessible to the isolated antibodies/peptides whereas those isolated by panning against pure antigens may recognize epitopes which are naturally buried in the membrane or blocked by carbohydrate modification. However, antibodies/peptides isolated with this method usually have a low to moderate affinity to their antigens/ligands.
Since each M13 phage particle presents five copies of the minor coat protein pIII, a phage particle displaying an antibody fragment on all copies of pIII can be considered a pentavalent antibody. This multivalent display of antibody fragments on phage greatly increases the avidity of the antibody and facilitates both screening and evaluation of phage antibodies. Isolated antibody fragments (scFvs or sdAbs) or peptides bind antigen much less efficiently since they exist primarily in a monovalent form and lack avidity.
An antibody fragment oligomerization strategy that permits pentavalency as in pIII phage display is the subject of PCT/CA02/01829 (MacKenzie and Zhang). Fusion of a single domain antibody (sdAb) to the homo-pentamerization domain of the B subunit of verotoxin (VT1B) results in the simultaneous pentamerization of the sdAb. The pentavalent sdAbs, termed pentabodies, bind much more strongly to immobilized antigen than their monomeric counterparts. In the instance of peptide hormone-binding sdAb, pentamerization resulted in 103 to 104-fold improvement in binding to immobilized antigen.
It is an object of the invention to provide a single-domain antibody with affinity for lung carcinoma.